A typical microwave oven door has a metal frame comprising a microwave-blocking screen sandwiched between a pair of transparent panels. As shown in FIG. 3 of U.S. Pat. No. 3,843,859, the exterior transparent panel may be glass and the interior panel may be a relatively thin plastic film such as a polycarbonate film. A pair of epoxy adhesive layers bond the screen to the glass panel and, in turn, the polycarbonate film to the screen. According to column 3, lines 8-12, the adhesive layers can either be coextensive with the screen or only extend around the border. Prior to the present invention, most microwave oven windows being produced employed a pair of pressure-sensitive adhesive layers around the border.
A window of especially economical construction employs an exterior glass panel and a coextensive conductive metal panel which has a central, perforated area that functions as a window with a microwave-blocking screen. An area of the metal panel somewhat larger than the perforated window area is indented away from the interior of the oven, and a transparent plastic film is adhered to the indented area by a pressure-sensitive adhesive transfer tape extending along the periphery of the plastic film. Because the plastic film is virtually the same size as the indented area and the metal panel is indented to a somewhat greater extent than the total thickness of the plastic film and the adhesive, the edges of the plastic film are protected from being accidentally caught during the movement of articles through the oven doorway. The mismatch in coefficient of thermal expansion between the metal panel and the plastic film tends to warp the plastic film. While the resultant optical distortion may be minor, this mars the esthetic appearance and is interpreted as a mark of low quality. To minimize this, the plastic film is made semi-rigid by being fairly thick, usually at least 20 mils. Because polyester film is not readily available in that thickness, polycarbonate film has almost always been used, even though it has rather high water-vapor transmission.
One advantage of that construction is that leakage of microwaves is minimized because the interior surface of the metal panel contacts the frame of the door. In a microwave oven door wherein the microwave-blocking screen covers the full door and an interior plastic panel is coextensive with the screen, there may be a path for leakage through that panel.